1. Field of the Invention
The present invention relates to a resin for toner. In particular, the invention concerns a resin for toner having improved offset resistance and long life as well as excellent productivity.
2. Description of the Prior Art
In recent years, along with propagation of copying machines or laser beam printers using toners amount of toners used therein has been increased rapidly, and there has been a strong demand in the market for developing toners which can be prepared in high productivity at low cost.
However, the production steps for toner is extremely complicated, requiring preliminary mixing of a binder resin and additives, melt-kneading, coarse pulverization, fine pulverization and sieving. Since the pulverization steps are rate-determining steps of the process for preparing the toner, development of a resin that can be pulverized easily in the pulverization steps has been highly desired. Development has also been making for a resin of satisfactory pulverization property (pulverizability) and, it has been disclosed in, for example, Japanese Patent Laid Open 195681/1987 to reduce molecular weight of the resin or to lower cross linking density thereby lowering melt viscosity of the resin.
However, toners prepared by these methods present a so-called offset phenomenon in which toner particles are stuck to the surface of a fixing roller upon fixing the toners to paper sheets, or toner-life is worsened in which succeeding copy sheets are contaminated, mechanical abrasion resistance of the toners is lost and toner characteristics are worsened in the copying machine after copying tens of thousands of sheets.
During the course of a study for preparing toners free from such disadvantage, it has been found that if the content of fine particles contained in a resin for toner is controlled to less than a certain amount when preparing the resin by suspension polymerization, the pulverizability of the toner can be improved and, accordingly, if the softening point of the resin is increased to such an extent as causing no offset phenomenon, the toner life can also be improved without worsening the fixing property so much.